Methods and devices for detecting open and/or shorts circuits in mems micro-mirror devices

ABSTRACT

According to the present invention there is provided methods and devices for detecting open and/or short circuits in MEMS micro-mirror devices, which use relative comparisons of voltage levels within the MEMS micro-mirror devices for detecting the occurrence of open and/or short circuits.

FIELD OF THE INVENTION

The present invention concerns methods and devices for detecting openand/or short circuits in MEMS micro-mirror devices, which use relativecomparisons of voltage levels within the MEMS micro-mirror devices fordetecting open and/or short circuits.

DESCRIPTION OF RELATED ART

A MEMS micro-mirror device is a device that contains an optical MEMS(Micro-Electrical-Mechanical-System). The optical MEMS micro-mirrordevice may comprise an elliptical, cylindrical, rectangular, square orrandom shape micro-mirror that is adapted to move and to deflect lightover time. The micro-mirror is typically connected by torsional arms toa fixed part and can tilt and oscillate along one or two axis. Forexample it can oscillate vertically and horizontally. Differentactuation principles can be used, including electrostatic, thermal,electro-magnetic or piezo-electric. MEMS micro-mirror devices are knownin which the area of these micro-mirrors are around a few mm². In thiscase, the dimensions of the MEMS micro-mirror device, comprising thepackaging, is around ten mm². This MEMS micro-mirror device is usuallymade of silicon, and can comprise a package that can include the drivingactuation electronics. Various optical components, such as for examplelenses, beam combiner, quarter-wave plates, beam splitter and laserchips, are assembled with the packaged MEMS to build a complete system.

A typical application of the MEMS micro-mirror devices is for opticalscanning and projection systems. In a projection system, a 2-D or 3-Dimage or video can be displayed on any type of projection surface. In acolour system, each pixel of the image is generated by combiningmodulated red, green and blue laser light, by means of, for example, abeam combiner, to generate a combined light beam which defines a pixelof the image or video. The MEMS micro-mirror in the MEMS micro-mirrordevice directs the combined light beam to a projection surface where thepixel of the image or video is displayed. Successive pixels of the imageor video are display in this manner. By means of its oscillations, theMEMS micro-mirror within the MEMS micro-mirror device will continuouslyscan the combined light beam from left to right and from top to bottom(or according to a different trajectory including e.g. Lissajoutrajectories) so that all the pixels of the image, or video, aredisplayed on the projection surface, successively, pixel-by-pixel. TheMEMS micro-mirror will oscillate about its oscillation axes at afrequency which ensures that the combined light beam is scanned acrossthe projection surface at such a speed that a complete image is visibleto a person viewing. A projection device which uses a MEMS micro-mirrorto scan light across a projection surface is commonly referred to as aMEMS micro-mirror projection device.

Typically, the MEMS micro-mirror in a MEMS micro-mirror projectiondevice is able to oscillate along a single oscillation axis. Therefore,in order to display a 2-D image on a screen a projection system willrequire two MEMS micro-mirrors; a first MEMS micro-mirror which isrequired to scan the combined light beam along the horizontal and asecond MEMS micro-mirror which is required to scan the combined lightbeam along the along the vertical. Alternatively the MEMS micro-mirrorin a MEMS micro-mirror projection device could be configured such thatit can be oscillated about two orthogonal oscillation axes.

Referring now to FIGS. 1a and 1 b which show a MEMS micro-mirrorcomponent 1 of a MEMS micro-mirror projection device. FIG. 1a provides aplan view of the MEMS micro-mirror component 1 and FIG. 1b shows a crosssectional view of the MEMS micro-mirror component1, taken along A-A′ ofFIG. 1 a.

The MEMS micro-mirror component 1 of the MEMS micro-mirror projectiondevice comprises a first support frame 2. A first torsional arm 3 a andsecond torsional arm 3 b connect a MEMS micro-mirror 4 to the supportframe 2. In this embodiment the support frame 2 is fixed (i.e.immovable). The first and second torsional arms 3 a,b define a firstoscillation axis 7 for the MEMS micro-mirror 4. A first conduction coil5 is supported on, and connected to, the MEMS micro-mirror 4. The firstconduction coil 5 is arranged to extend, from a first electrical contact9 a which is located on the support frame 2, along the first torsionalarm 3 a, around the perimeter of the MEMS micro-mirror 4 and back alongthe first torsional arm 3 a to a second electrical contact 9 b which islocated on the support frame 2. In the MEMS micro-mirror component 1 theconduction coil 5 is shown to be arranged to have one turn on MEMSmicro-mirror 4; it will be understood that the conduction coil 5 mayextend around the MEMS micro-mirror 4 any number of times so as todefine any number of turns on the MEMS micro-mirror 4.

Collectively, the first support frame 2, first and second torsional arms3 a,b and the MEMS micro-mirror 4, and first conduction coil 5, definecollectively what is referred to as a MEMS die 10. As shown in FIG. 1bthe MEMS die 10 is arranged in cooperation with a magnet 6 such thefirst conduction coil 5 is submerged in the magnetic field ‘B’ generatedby the magnet 6.

During use an electric current ‘I’ is passed through the firstconduction coil 5. As the first conduction coil 5 is submerged in themagnetic field ‘B’ created by the magnet 6, the conduction coil 5 willprovide a Laplace force which will be applied to the MEMS micro-mirror4. The Laplace force will cause the MEMS micro mirror 4 to move aboutits first oscillation axis 7. The electric current ‘I’ which is passedthrough the first conduction coil 5 is configured for example to besinuous or square, so that the MEMS micro-mirror 4 is continuouslyoscillated about its first oscillation axis 7. If the MEMS micro-mirror4 reflects light as it is oscillating about the first oscillation axes7, light reflected by the MEMS micro-mirror 4 will be scanned along thehorizontal or vertical. This will, for example, enable combined lightbeams which the MEMS micro mirror 4 receives, to be scanned across theprojection screen.

It should be understood that the MEMS micro-mirror component 1 couldalternatively comprise a MEMS micro-mirror which can oscillate about twoorthogonal oscillation axes; in such a case the MEMS micro mirrorcomponent 1 will typically comprise a second conduction coil. Such aMEMS micro-mirror component 1 could scan light in two dimensions (e.g.along the horizontal and vertical) when the conduction coils conductcurrent.

If a short circuit or open circuit occurs in the conduction coil 5 whichdrives the MEMS micro-mirror 4 to oscillate about the oscillation axes7, then the MEMS micro-mirror 4 will stop oscillating; as a result lightreflected by the MEMS micro-mirror 4 would no longer be scanned butwould be concentrated along a single direction to a single point on thedisplay screen. The high concentration of light which is reflected inthe single direction can damage eyes of person. Accordingly, is itnecessary to quickly detect the occurrence of open or short circuits inMEMS mirror projection devices so that the MEMS mirror projectiondevices can be quickly shut-down in the event of an open or shortcircuit, so as to avoid damaging a person's eyes.

Typically the detection of open or short circuits in MEMS micro-mirrorprojection devices is achieved by measuring the voltage which is acrossthe MEMS micro-mirror 4 and comparing the measured voltage to areference voltage. The voltage across the MEMS micro-mirror 4 willincrease in the case of an open or short circuit; thus if the measuringthe voltage across the MEMS micro-mirror 4 exceeds the reference voltagethen a short or open circuit is detected. When a short or open circuitis detected the MEMS micro-mirror projection device is usuallyautomatically shut-down so as to avoid damaging the eyes of a person.However, the voltage across the MEMS micro-mirror 4 and the referencevoltage are each affected by temperature; the temperature of MEMSmicro-mirror projection device will increase during use and thisincrease in temperature affects the voltage across the MEMS micro-mirror4 and the reference voltage. Accordingly, the measured voltage acrossthe MEMS micro-mirror 4 may exceed the reference voltage due to changesin temperature alone and not due to the occurrence of an open or shortcircuit. Thus, current methods is can erroneously indicate theoccurrence of an open or short circuit in the MEMS micro-mirrorprojection device, which leads to unnecessary shut-down of the device.

Another approach to the detection of open or short circuits in MEMSmicro-mirror projection devices involves the use of PLL (i.e. a phaselocked loop, which is a circuit that provides constant phase shiftbetween two signals). However the problem with this approach is that itis slow to detect an open or short circuit. Therefore, there is a longperiod between the actual occurrence of the open or short circuit inMEMS micro-mirror projection device and the shutting-down of the MEMSmirror projection device. There is a high risk that the eyes of a personbecome damaged during in the period between the actual occurrence of theopen or short circuit and the shutting-down of the MEMS micro-mirrorprojection device.

It is an aim of the present invention to obviate or mitigate at leastsome of the above-mentioned disadvantages.

BRIEF SUMMARY OF THE INVENTION

According to the invention, these aims are achieved by means of a MEMSmirror device comprising, a H bridge circuit; a MEMS mirror which isconnected as a load to the H bridge circuit so that voltage can beselectively applied across the MEMS mirror in either direction; and acurrent source which is connected to the H bridge circuit; wherein theMEMS mirror device further comprises, a means for comparing the voltagesat a first and second side of the MEMS mirror to a voltage across thecurrent source, so that an open circuit in the MEMS mirror device can bedetected.

The H bridge circuit and/or the comparator and/or any other circuitblocks or features, and the MEMS mirror may be provided on separate,mechanically independent, silicon wafers or may be provided on the samesilicon wafer.

Advantageously the present invention compares relative levels in thedevice in order to detect the occurrence of a short or open circuit. Thedetection of a short or open circuits therefore independent oftemperature changes in the device, changes in bias currents, changes inresistances. Accordingly the occurrence of a short or open circuit canbe detected more accurately.

The first and second sides of the MEMS mirror are preferably oppositesides of the MEMS mirror where an actuation coil on the MEMS mirror iselectrically connected to the H bridge circuit. A actuation coil is acoil which can conduct current to effect oscillation of the MEMS mirrorabout one or more oscillation axes.

The means for comparing may comprise an IC circuit

The means for comparing may comprise an Analogue-to-Digital converter.

The current source may comprise a first and second transistor connectedin series.

The means for comparing may comprise,

a first comparator;

a second comparator; and

an OR gate;

wherein outputs of the first and second comparator are arranged to beinput to the OR gate, and

wherein the voltage at the first side of the MEMS mirror and the voltageacross the current source are inputs to the first comparator and thevoltage at the second side of the MEMS mirror and the voltage across thecurrent source are inputs to the second comparator.

The current source may comprise a first and second transistor connectedin series and the means for comparing is configured to compare voltagesat the first and second sides of the MEMS mirror to a voltage at ajunction between the first and second transistors.

The means for comparing may comprise,

a first comparator;

a second comparator; and

an OR gate;

wherein outputs of the first and second comparator are arranged to beinput to the OR gate, and

wherein the voltage at the first side of the MEMS mirror and the voltageat the junction between the first and second transistors are inputs tothe first comparator and the voltage at the second side of the MEMSmirror and the voltage at the junction between the first and secondtransistors are inputs to the second comparator.

According to a further aspect of the present invention there is provideda MEMS mirror device comprising,

a H bridge circuit;

a MEMS mirror which is connected as a load to the H bridge circuit sothat voltage can be selectively applied across the MEMS mirror in eitherdirection; and

a current source which is connected to the H bridge circuit whereincurrent source comprises a first and second transistor connected inseries; wherein the MEMS mirror device further comprises,

a means for comparing which is configured to compare voltage across thefirst and second transistors to voltage at a junction between the firstand second transistors to detect an open circuit.

The means for comparing may comprise a single comparator wherein thevoltage across the first and second transistors and the voltage at ajunction between the first and second transistors are inputs to thesingle comparator.

According to a further aspect of the present invention there is provideda method for detecting an open circuit in the MEMS mirror devicecomprising, a H bridge circuit; a MEMS mirror which is connected as aload to the H bridge circuit so that voltage can be selectively appliedacross the MEMS mirror in either direction; and a current source whichis connected to the H bridge circuit; wherein the MEMS mirror devicefurther comprises, a means for comparing the voltages at a first andsecond side of the MEMS mirror to a voltage across the current source,so that an open circuit in the MEMS mirror device can be detected, themethod comprising the step of,

comparing voltages at first and second sides of the MEMS mirror to avoltage across the current source,

detecting a change in result of the comparison to indentify theoccurrence of an open circuit in the MEMS mirror device.

The step of comparing voltages at first and second sides of the MEMSmirror to a voltage across the current source may comprise,

comparing the voltage at the first side of the MEMS mirror to thevoltage across the current source using a first comparator, and,

comparing the voltage at the second side of the MEMS mirror to thevoltage across the current source using a second comparator, and,

passing the output of the first and second comparators through a ORgate; and

wherein the step of detecting a change in the result of the comparisonto indentify the occurrence of an open circuit in the MEMS mirror devicecomprises,

detecting a change in the output of the OR gate.

In any of the above-mentioned methods, the current source in said MEMSmirror device comprises a first and second transistor connected inseries, and

the step of comparing voltages at first and second sides of the MEMSmirror to a voltage across the current source may comprise,

comparing the voltage at the first side of the MEMS mirror to a voltageat a junction between the first and second transistors using a firstcomparator, and,

comparing the voltage at the second side of the MEMS mirror to thevoltage at a junction between the first and second transistors using asecond comparator, and,

passing the output of the first and second comparators through a NORgate; and, wherein the step of detecting a change in the result of thecomparison to indentify the occurrence of an open circuit in the MEMSmirror device comprises,

detecting a change in the output of the NOR gate.

The method may comprise the steps of

comparing voltage across the first and second transistors to voltage ata junction between the first and second transistors to detect an opencircuit using a comparator; and

wherein the step of detecting a change in result of the comparison toindentify the occurrence of an open circuit in the MEMS mirror devicecomprises,

detecting a change in the output of the comparator.

According to a further aspect of the present invention there is provideda MEMS mirror device comprising,

a H bridge circuit;

a MEMS mirror which is connected as a load to the H bridge circuit sothat voltage can be selectively applied across the MEMS mirror in eitherdirection; and

a current source which is connected to the H bridge circuit; wherein theMEMS mirror device further comprises

a means for comparing the voltages at a first and second side of theMEMS mirror, so that a short circuit in the MEMS mirror device can bedetected.

The first and second sides of the MEMS mirror are preferably oppositesides of the MEMS mirror where an actuation coil on the MEMS mirror iselectrically connected to the H bridge circuit. An actuation coil is acoil which can conduct current to effect oscillation of the MEMS mirrorabout one or more oscillation axes.

The means for comparing may comprise an IC circuit

The means for comparing may comprise an Analogue-to-Digital converter.

The means for comparing may comprise,

a first comparator;

a second comparator; and

an XNOR gate;

wherein outputs of the first and second comparator are arranged to beinput to the XNOR gate, and

wherein the voltage at the first side of the MEMS mirror and the at thevoltage at the second side of the MEMS mirror are inputs to the firstcomparator and the voltage at the first side of the MEMS mirror and theat the voltage at the second side of the MEMS mirror are inputs to thesecond comparator.

The means for comparing may comprise,

a single XNOR gate wherein a ground pin of the XNOR gate is electricallyconnected so as to have the same potential as the voltage across thecurrent source, and

wherein the voltage at the first side of the MEMS mirror is a firstinput to the XNOR gate and the voltage at the second side of the MEMSmirror is a second input to the XNOR gate.

Preferably the ground pin of the XNOR gate is electrically connected toa junction which is between the H-bridge and the current source.

According to a further aspect of the present invention there is provideda method for detecting a short circuit in the MEMS mirror device whichcomprise, a H bridge circuit; a MEMS mirror which is connected as a loadto the H bridge circuit so that voltage can be selectively appliedacross the MEMS mirror in either direction; and a current source whichis connected to the H bridge circuit; wherein the MEMS mirror devicefurther comprises a means for comparing the voltages at a first andsecond side of the MEMS mirror, so that a short circuit in the MEMSmirror device can be detected, the method comprising the step of,

comparing the voltages at the first and second side of the MEMS mirror,so that a short circuit in the MEMS mirror device can be detected,

detecting a change in result of the comparison to indentify theoccurrence of a short circuit in the MEMS mirror device.

The step of comparing the voltages at the first and second side of theMEMS mirror, so that a short circuit in the MEMS mirror device can bedetected, may comprise,

comparing the voltage at the first side of the MEMS mirror to thevoltage at the second side of the MEMS mirror, using a first comparator,and,

comparing the voltage at the first side of the MEMS mirror to thevoltage at the second side of the MEMS mirror, using a secondcomparator, and,

passing the output of the first and second comparators through an XNORgate; and wherein the step of detecting a change in the result of thecomparison to indentify the occurrence of a short circuit in the MEMSmirror device comprises,

detecting a change in the output of the XNOR gate.

The step of comparing the voltages at the first and second side of theMEMS mirror, so that a short circuit in the MEMS mirror device can bedetected, may comprise,

providing the voltage at the first side of the MEMS mirror as a firstinput to a single XNOR gate which has its ground electrically connectedso as to have the same potential as the voltage across the currentsource;

providing the voltage at the second side of the MEMS mirror as a secondinput to the single XNOR gate; and

wherein the step of detecting a change in the result of the comparisonto indentify the occurrence of a short circuit in the MEMS mirror devicecomprises,

detecting a change in the output of the XNOR gate.

According to a further aspect of the present invention there is provideda MEMS mirror device comprising,

a H bridge circuit;

a MEMS mirror which is connected as a load to the H bridge circuit sothat voltage can be selectively applied across the MEMS mirror in eitherdirection; and

a current source which is connected to the H bridge circuit; wherein theMEMS mirror device further comprises

a means for comparing voltage across the current source to an referencevoltage to detect a short circuit in the MEMS mirror device.

The reference voltage may be an average value for the voltage across acurrent source over a predefined period of time.

The means for comparing may comprise,

a comparator; and

a low pass filter which is electrically connected to a junction betweenthe H bridge and current source;

-   -   wherein a first input of the low pass filter is electrically        connected to a junction between the H bridge and current source        so that the first input to the comparator is at a voltage which        is equal to the voltage across the current source, and wherein a        second input to the comparator is electrically connected to low        pass filter so that the second input to the comparator is at a        voltage which is equal to the voltage across the low pass        filter.

The low pass filter can store previous voltages across the currentsource. The voltage across the low pass filter is therefore the averagevalue for the voltage across a current source over a predefined periodof time. The low pas filter may comprise one or more capacitors.

The means for comparing further may comprise a buffer which iselectrically connected between said junction and capacitor.

The means for comparing further may comprise a means for providing anoffset to the average value for the voltage across a current source.

A method for detecting a short circuit in the MEMS mirror device whichcomprises a H bridge circuit; a MEMS mirror which is connected as a loadto the H bridge circuit so that voltage can be selectively appliedacross the MEMS mirror in either direction; and a current source whichis connected to the H bridge circuit; wherein the MEMS mirror devicefurther comprises a means for comparing voltage across the currentsource to an reference voltage to detect a short circuit in the MEMSmirror device, the method comprising the step of,

comparing voltage across the current source to an reference voltage;

detecting a change in result of the comparison to indentify theoccurrence of a short circuit in the MEMS mirror device.

The reference voltage may be an average value for the voltage across acurrent source over a predefined period of time.

The method may comprise the step of storing voltage across the currentsource using a low pass filter. Storing voltage across the currentsource in the low pass filter ensures that the voltage across the lowpass filter is representative of the average value for the voltageacross a current source over a predefined period of time.

The method may comprise the step of comparing the voltage across the lowpass filter to a comparator which is used to compare the voltage acrossthe current source to an average value for the voltage across a currentsource, using a buffer.

The method may comprise the step of applying an offset to the averagevalue for the voltage across a current source. Preferably the offset isapplied before the voltage across the current source to an average valuefor the voltage across a current source.

Preferably the offset is applied by the comparator. Preferably thecomparator comprises an intrinsic offset at its input.

The means for comparing voltage across the current source to anreference voltage to detect a short circuit in the MEMS mirror devicemay comprise,

an analogue to digital converter which is electrically connected to ajunction between the H bridge and current source so that the analogue todigital converter can convert the voltage across the current source to adigital value; and

a comparator which is arranged to compare the digital value for thevoltage across the current source to a reference digital value which isrepresentative of said reference voltage.

The step of comparing voltage across the current source to an referencevoltage may comprise, converting the voltage which is across the currentsource from an analogue signal to a digital value, and comparing thedigital value to reference digital value which is representative of saidreference voltage.

The means for comparing may comprise a first and second comparator,wherein a first input of the first comparator is electrically connectedto a junction between the H bridge and current source so that the firstinput has a voltage equal to the voltage across the current source, anda second input of the first comparator is provided with a firstreference voltage; and

wherein a second input of the second comparator is electricallyconnected to a junction between the H bridge and current source so thatthe second input has a voltage equal to the voltage across the currentsource, and a first input of the second comparator is provided with asecond reference voltage.

The means for comparing may comprise a first, second and third resistor,which are connected in series, and voltage is applied across the first,second and third resistors such that the voltage at a junction betweenthe first and second resistors define the first reference voltage andthe voltage at a junction between the second and third resistors definethe second reference voltage.

The means for comparing may comprise a first and second digital toanalogue convertor, wherein the first and second digital to analogueconvertors can receive first and second digital reference inputsrespectively, and wherein the output of the first digital convertordefines the first reference voltage and the output of the first digitalconvertor defines the second reference voltage.

A MEMS mirror device may further comprise a controller which can shutoff the MEMS mirror device if an open circuit and/or short circuit isdetected.

It will be understood that in each of the above-mentioned embodiment afirst input of a comparator will be a positive input node of thecomparator and a second input of that comparator will be a negativeinput node of the comparator. Or, the first input of a comparator willbe a negative input node of the comparator and a second input of thatcomparator will be a positive input node of the comparator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof embodiments of the present invention, which are given by way ofexample only, and illustrated by the figures, in which:

FIG. 1a shows an aerial view of a known MEMS micro-mirror device; FIG.1b shown a cross sectional view of the MEMS micro-mirror device shown inFIG. 1a along A-A′;

FIG. 2a shows a H-bridge with a MEMS mirror connected as a load to the Hbridge, and a current source; FIG. 2b shows voltages at points A, B, Cand D of FIG. 2 a;

FIG. 3 shows voltages at points A, B, C and D of FIG. 1a in the case ofan open circuit occurring in an conduction coil of the MEMSmicro-mirror;

FIG. 4 shows voltages at points A, B, C and D of FIG. 1a in the case ofa short circuit occurring in a conduction coil of the MEMS micro-mirror;

FIG. 5 shows a MEMS mirror device according to a first embodiment of thepresent invention, in which an open circuit can be detected;

FIG. 6 shows a MEMS mirror device according to a further embodiment ofthe present invention in which an open circuit can be detected;

FIG. 7 shows a MEMS mirror device according to a further embodiment ofthe present invention in which an open circuit can be detected;

FIG. 8 shows a MEMS mirror device according to a further embodiment ofthe present invention in which a short circuit can be detected;

FIG. 9 shows a MEMS mirror device according to a further embodiment ofthe present invention in which a short circuit can be detected;

FIG. 10 shows a MEMS mirror device according to a further embodiment ofthe present invention in which a short circuit, as well as a partialshort circuit, can be detected;

FIG. 11 shows a MEMS mirror device according to a further embodiment ofthe present invention in which a short circuit, as well as a partialshort circuit, can be detected;

FIG. 12 shows a MEMS mirror device according to a further embodiment ofthe present invention in which a short and open circuit can be detected.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

FIG. 2a shows a circuit 20 which comprises a H-bridge circuit 21 with aMEMS micro-mirror 22 connected as a load to the H bridge circuit 21 sothat voltage can be selectively applied across the MEMS micro-mirror 22in either direction.

As is known in the art the H-bridge circuit 21 comprises a firstp-transistor (P_(A)) electrically connected in series with a firstn-transistor (N_(A)), and a second p-transistor (P_(B)) electricallyconnected in series with a second n-transistor (N_(B)). The first p- andn-transistors (P_(A),N_(A)) are electrically connected in parallel tothe second p- and n-transistors (P_(B),N_(B)).

The MEMS micro-mirror 22 connected as a load to the H bridge circuit 21by mean of a conduction coil 28. The conduction coil 28 cooperates withthe MEMS micro-mirror 22 and can conduct current in a magnetic field soas to generate a Lorentz force which can oscillate the MEMS micro-mirror22 about an oscillation axis. The conduction coil 28 therefore actuatesthe MEMS micro-mirror 22 to oscillate about an oscillation axis byconducting current. The conduction coil 28 is electrically connected tothe H-bridge circuit 21 at opposite sides (first and second sides) ofthe MEMS micro-mirror 22, at points A and B.

The top 26 of the H-bridge circuit 21 is electrically connected to apower source (V_(DD)) and the bottom 24 of the H-bridge circuit 21 iselectrically connected to a current source 23 which is grounded (gnd).The H-bridge circuit 21 is electrically connected to a current source 23at point C. The current source 23 comprises a first and secondn-transistors 25 a,b which are connected in series; the junction betweenthe first and second n-transistors 25 a,b is point D.

FIG. 2b shows voltages at the points A, B, C and D of the circuit 20shown in FIG. 2a , when the circuit 20 is in normal operation. Duringnormal operation current will pass either of two routes; the first routeis from power source (V_(DD)), through the first p-transistor (P_(A)),through the conduction coil 28 from the left to the right hand side ofthe MEMS micro-mirror 22, through the second n-transistor (N_(B)) andthrough the current source 23 to ground (gnd); and the second route isfrom power source (V_(DD)), through the second p-transistor (P_(B)),through the conduction coil 28 from the right to the left hand side ofthe MEMS micro-mirror 22, through the first n-transistor (N_(A)) andthrough the current source 23 to ground (gnd). For both the first andsecond routes, a voltage drop (U_(m)) will occur across the MEMSmicro-mirror 22 as the current passes through the conduction coil 28;therefore the voltage at A and B will always be opposite in phase (i.e.A and B can have only two values: Vdd or Vdd−Um; so if A is at Vdd, Bwill be at Vdd−Um, and if A is at Vdd−Um, B will be at Vdd) when currentfollows along the first route then the voltage at A is equal to Vdd andthe voltage at B will be is equal to Vdd−Um; when current follows alongthe second route then the voltage at A is equal to Vdd−Um the voltage atB will be is equal to Vdd, as is illustrated in the graphs of FIG. 2 b.

It should also be noted that the voltages which are at A and B willalways be greater than the voltage which is at C (V_(HR)) i.e.(Vdd−Um)>V_(HR), and Vdd>V_(HR). Furthermore, the voltage at C (V_(HR))will always be greater than the voltage at D, due to the voltage dropwhich occurs across the second n-transistor 25 b of the current source23.

FIG. 3 shows voltages at points A, B, C and D of FIG. 1a in the case ofan open circuit occurring in the conduction coil 28 of the MEMSmicro-mirror 22. When an open circuit occurs the voltage drop across theMEMS micro-mirror 22 becomes V_(DD) (i.e. Um is equal to +V_(DD) or−V_(DD) depending on whether the current is passing along the first orsecond routes through the H-bridge); thus when the voltage at A isV_(DD) the voltage at B is ‘0’ and when the voltage at B is V_(DD) thevoltage at A is ‘0’. Accordingly, the current through the current source23 goes to ‘0’, and importantly, the voltage at C becomes equal to thevoltage at D, and is equal to ‘0’, as is illustrated in the graphs ofFIG. 3.

FIG. 4 shows voltages at points A, B, C and D of FIG. 1a in the case ofa short circuit occurring in the conduction coil 28 of the MEMSmicro-mirror 22. As mentioned with reference to FIG. 2b , during normaloperation, the voltage at A and B will always be opposite in phase (i.e.A and B can have only two values: Vdd or Vdd−Um; so if A is at Vdd, Bwill be at Vdd−Um, and if A is at Vdd−Um, B will be at Vdd); whencurrent follows along the first route then the voltage at A is equal toVdd and the voltage at B will be is equal to Vdd−Um; when currentfollows along the second route then the voltage at A is equal to Vdd−Umthe voltage at B will be is equal to Vdd. However, when a short circuitoccurs in the conduction coil 28 of the MEMS micro-mirror 22, A and Bwill have the same potential; specifically when a short circuit occursthe voltage at A will be equal to Vdd and the voltage at B will be isequal to Vdd, as is illustrated in the graphs of FIG. 4.

FIG. 5a shows a MEMS micro-mirror device 50 according to a firstembodiment of the present invention, in which an open circuit can bedetected. The MEMS micro-mirror device 50 comprises a circuit 51 whichhas all of the features of the circuit 20 shown in FIG. 2a and likefeatures are awarded the same reference numbers.

The MEMS micro-mirror device 50 further comprises a means for comparing52, which is configured to compare the voltages at first and secondsides 58 a,58 b of the MEMS micro-mirror 22 to the voltage across thecurrent source 23 (V_(HR)), so that an open circuit in the MEMS mirrordevice 50 can be detected. Specifically, the means for comparing 52,compares the voltages at points A and B to the voltage across thecurrent source 23 (V_(HR)), so that an open circuit in the MEMS mirrordevice 50 can be detected.

The means for comparing 52 comprises an IC circuit which comprises afirst comparator 53 which has an output (E); a second comparator 54which has an output (F); and a OR gate 55 which has an output (G).Outputs (E,F) of the first and second comparators 54 are arranged to beinput to the OR gate 55. The voltage at the first side 58 a of the MEMSmicro-mirror 22 (i.e. the voltage at point A) and the voltage across thecurrent source 23 (V_(HR)) (i.e. the voltage at point C), are inputs tothe first comparator 53, and the voltage at the second side 58 b of theMEMS micro-mirror 22 (i.e. the voltage at point B) and the voltageacross the current source 23 (V_(HR)) (i.e. the voltage at point C), areinputs to the second comparator 54. A voltage offsetting means 59 isfurther provided for providing an voltage offset (Uoff) to the voltageacross the current source 23 (V_(HR)) (i.e. the voltage at point C)before it is input to the first and second comparators 23 respectively.

FIG. 5b show the voltages at points A-C and the Boolean values atoutputs E-G, of the MEMS micro-mirror device 50 shown in FIG. 5a when anopen circuit occurs. At time t_(o) the MEMS micro-mirror device 50undergoes normal operation. During normal operation at least one ofvoltages at points A and B will always be greater than the voltageacross the current source 23 (V_(HR)) (i.e. the voltage at point C) plusthe voltage offset (Uoff) which is the input offset of the comparator(i.e. at least one of voltages at points A and B will always be greaterthan ‘V_(HR)+Uoff’); accordingly the Boolean values at the outputs E andF will be ‘0’ and the Boolean value at the output of the OR gate 55 willbe ‘0’. The voltage offset (Uoff) is built by a size asymmetry betweentransistors of the comparator input.

At time t₁ an open circuit occurs in the conduction coil 28 of the MEMSmicro-mirror 22 which causes the MEMS micro-mirror 22 to stoposcillating. The voltage across the current source 23 (V_(HR)) (i.e. thevoltage at point C) will go to ‘0’, and the voltages at points A and Bwill alternately equal V_(DD) and ‘0’ as shown. Accordingly, thevoltages at points A and B will alternately be smaller than the voltageacross the current source 23 (V_(HR)) (i.e. the voltage at point C) plusthe voltage offset (Uoff) which is the input offset of the comparator(i.e. the voltages at points A and B will alternately be smaller than‘V_(HR)+Uoff’) causing the Boolean values at outputs E and F to bealternately be 1 and 0, as shown in FIG. 5b . Accordingly the Booleanvalue at the output G of the OR gate 55 will go to ‘1’ when an opencircuit occurs in the conduction coil 28. The detection of an opencircuit can be achieved by monitoring the output G of the OR gate 55 forwhen its Boolean value changes from ‘0’ to ‘1’.

The MEMS micro-mirror device 50 may further comprise a control unitwhich is operatively connected to the output G of the OR date 55, and isconfigured to automatically shut-down the MEMS micro-mirror device 50when the output G of the OR gate 55 changes from ‘0’ to ‘1’.

FIG. 6a shows a MEMS mirror device 60 according to a further embodimentof the present invention in which an open circuit can be detected. TheMEMS mirror device 60 has many of the same features as the MEMS mirrordevice 50 shown in FIG. 5a , and like features are awarded the samereference numbers. The MEMS micro-mirror device 60 comprises a circuit61 which has all of the features of the circuit 20 shown in FIG. 2a andlike features are awarded the same reference numbers.

The difference between the MEMS mirror device 60 in FIG. 6a and the MEMSmirror device 50 in FIG. 5a is that in the MEMS mirror device 60 thevoltage at point D (i.e. the voltage at the junction between the firstand second n-transistors 25 a,b of the current source 23) is used forthe comparison. Specifically, the MEMS mirror device 60 comprises ameans for comparing 62 which is configured to compare the voltage at thefirst side 58 a of the MEMS micro-mirror 22 (i.e. the voltage at pointA) to the voltage at the junction between the first and secondn-transistors 25 a,b of the current source 23 (i.e. the voltage at pointD), and for comparing the voltage at the second side 58 b of the MEMSmicro-mirror 22 (i.e. the voltage at point B) to the voltage at thejunction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D).

The means for comparing 62 comprises an IC circuit which comprises afirst comparator 63 which has an output (E); a second comparator 64which has an output (F); and a OR gate 65 which has an output (G).Outputs (E,F) of the first and second comparators 63,64 are arranged tobe input to the OR gate 65. The voltage at the first side 58 a of theMEMS micro-mirror 22 (i.e. the voltage at point A) and the voltage atthe junction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D), are inputs to the firstcomparator 63, and the voltage at the second side 58 b of the MEMSmicro-mirror 22 (i.e. the voltage at point B) and the voltage at thejunction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D), are inputs to thesecond comparator 64. The voltage offsetting means 69 if furtherprovided to provide an voltage offset (Uoff) to the voltage at thejunction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D) before it is input tothe first and second comparators 63,64 respectively.

FIG. 6b show the voltages at points A-D and the Boolean values atoutputs E-G, of the MEMS micro-mirror device 60 shown in FIG. 6a when anopen circuit occurs. At time t_(o) the MEMS micro-mirror device 60undergoes normal operation. During normal operation at least one ofvoltages at points A and B will always be greater than the voltage atthe junction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D) plus the voltage offset(Uoff) which is the input offset of the comparator (i.e. at least one ofvoltages at points A and B will always be greater than ‘Voltage at pointD+Uoff’); accordingly the Boolean values at the outputs E and F will be‘0’ and the Boolean value at the output of the OR gate 65 will be ‘0’.

At time t₁ an open circuit occurs in the conduction coil 28 of the MEMSmicro-mirror 22 which causes the MEMS micro-mirror 22 to stoposcillating. The voltage across the current source 23 (V_(HR)) (i.e. thevoltage at point C) and therefore also the voltage at the junctionbetween the first and second n-transistors 25 a,b of the current source23 (i.e. the voltage at point D), will go to ‘0’. The voltages at pointsA and B will alternately equal V_(DD) and ‘0’ as shown in FIG. 6b .Accordingly, the voltages at points A and B will alternately be smallerthan the voltage at point D plus the voltage offset (Uoff) which is theinput offset of the comparator causing the Boolean values at outputs Eand F to be alternately be 1 and 0, as shown in FIG. 6b . Accordinglythe Boolean value at the output G of the OR gate 65 will go to ‘1’ whenan open circuit occurs in the conduction coil 28. The detection of anopen circuit can be achieved by monitoring the output G of the OR gate65 for when its Boolean value changes from ‘0’ to ‘1’.

The MEMS micro-mirror device 60 may further comprise a control unitwhich is operatively connected to the output G of the OR gate 65, and isconfigured to automatically shut-down the MEMS micro-mirror device 60when the output G of the OR gate 65 changes from ‘0’ to ‘1’

The advantage of using the voltage at D for doing the comparison insteadof using the voltage at point C, is that when measuring at point C theminimum voltage between point A and Point C or minimum voltage betweenpoint B and Point C, is a few tens of millivolts only; which means thatto be able to compare the voltages at points A and C or the voltages atpoints B and C, the comparator internal offset has to be below few tensof millivolts, which is very difficult to achieve. On the other side,the voltage between C and D is around 200 mV, therefore the voltagebetween A and D or B and D will be around 200 mV instead of few tens ofmW. In that case one can design a comparator which has a more relaxedconstrain on its internal offset value, for example 100 mV, that willthen be easier and cheaper to design, as well as being more robust tomanufacturing tolerances. Indeed, a comparator which has low inputoffset value, for example less than 50 mV input offset at alltemperatures and process variations needs more silicon area and moreconsumption. As a result, the voltage between point A and point D(Va−Vd; respectively Vb−Vd) is higher than the voltage between point Aand point C (Va−Vc; respectively Vb−Vc) because with this solutionvoltage between point A and point D (Va−Vd) is effectively Va−Vc+Vc−Vd(respectively Vb−Vd is effectively Vb−Vc+Vc−Vd). So in this solution itis easier to design a comparator that compares higher signals, where thesolution enables to have the signal being a part of 50 mV more thanbefore.

FIG. 7a shows a MEMS micro-mirror device 70 according to a furtherembodiment of the present invention, in which an open circuit can bedetected. The MEMS micro-mirror device 70 comprises a circuit 71 whichhas all of the features of the circuit 20 shown in FIG. 2a and likefeatures are awarded the same reference numbers.

The MEMS micro-mirror device 70 further comprises a means for comparing72, which is configured to compare the voltage at the junction betweenthe first and second n-transistors 25 a,b of the current source 23 (i.e.the voltage at point D) to the voltage across the current source 23(V_(HR)), so that an open circuit in the MEMS mirror device 70 can bedetected.

The means for comparing 72 comprises an IC circuit which comprises asingle comparator 73 which has an output (E). The voltage at thejunction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D) and the voltage acrossthe current source 23 (V_(HR)) (i.e. the voltage at point C), are inputsto the single comparator 73. A voltage offsetting means 59 is furtherprovided for providing an voltage offset (Uoff) to the voltage at thejunction between the first and second n-transistors 25 a,b of thecurrent source 23 (i.e. the voltage at point D) before it is input tosingle comparator 73.

FIG. 7b show the voltages at points A-D and the Boolean values at outputE of the single comparator 73, of the MEMS micro-mirror device 70 shownin FIG. 7a when an open circuit occurs. At time t_(o) the MEMSmicro-mirror device 70 undergoes normal operation. During normaloperation the voltage at point C will always be greater than the voltageat point D plus the voltage offset (Uoff) which is the input offset ofthe comparator (i.e. the voltage at point C will always be greater than‘Voltage at point D+Uoff’); accordingly the voltages at points C and Dwill always differ and thus the Boolean value at the output E of thesingle comparator 73 will be ‘0’.

At time t₁ an open circuit occurs in the conduction coil 28 of the MEMSmicro-mirror 22 which causes the MEMS micro-mirror 22 to stoposcillating. The voltage across the current source 23 (V_(HR)) (i.e. thevoltage at point C) and therefore also the voltage at the junctionbetween the first and second n-transistors 25 a,b of the current source23 (i.e. the voltage at point D), will both go to ‘0’. The voltages atpoints C and D will therefore become equal and thus the Boolean value atthe output Eof the single comparator 73 will change from ‘0’ to ‘1’.Thus in the MEMS micro-mirror device 70 an open circuit can be detectedby monitoring for a change in the output E of the single comparator 73from ‘0’ to ‘1’.

The MEMS micro-mirror device 70 may further comprise a control unitwhich is operatively connected to the output E of the single comparator73, and which is configured to automatically shut-down the laser lightsource, and/or the laser driver and/or the MEMS micro-mirror device 70,when the output E of the single comparator 73 changes from ‘0’ to ‘1’.

FIG. 8 shows a MEMS mirror device 80 according to a further embodimentof the present invention in which a short circuit can be detected. TheMEMS micro-mirror device 80 comprises a circuit 81 which has all of thefeatures of the circuit 20 shown in FIG. 2a and like features areawarded the same reference numbers.

The MEMS micro-mirror device 80 further comprises a means for comparing82 which is configured to compare the voltages at first and second sides58 a, 58 b of the MEMS micro-mirror 22 to each other, so that an opencircuit in the MEMS mirror device 50 can be detected. Specifically, themeans for comparing 82, compares the voltage at point A to the voltage aB, so that an open circuit in the MEMS mirror device 80 can be detected.

The means for comparing 82 comprises an IC circuit which comprises afirst comparator 83 which has an output (H); a second comparator 84which has an output (I); and a XNOR gate 85 which has an output (J).Outputs (H,I) of the first and second comparators 83,84 are arranged tobe input to the XNOR gate 85. The voltage at the first side 58 a of theMEMS micro-mirror 22 (i.e. the voltage at point A) and the voltage atthe second side 58 b of the MEMS micro-mirror 22 (i.e. the voltage atpoint B) are first and second inputs to the first comparator 83respectively. The voltage at the second side 58 b of the MEMSmicro-mirror 22 (i.e. the voltage at point B) and the voltage at thefirst side 58 b of the MEMS micro-mirror 22 (i.e. the voltage at pointA) are first and second inputs to the second comparator 84 respectively.It will be understood that in this example, a first input is an input tothe positive node of a comparator and a second input is an input to thenegative node of a comparator; however it will be understood thatalternatively, a first input may be an input to the negative node of acomparator and a second input may an input to the positive node of acomparator.

A voltage offsetting means 89 is further provided for providing avoltage offset (Uoff) to the voltage at point A before it is input tothe first comparators 83 and for providing a voltage offset (Uoff) tothe voltage at point B before it is input to the second comparators 84.Alternatively the voltage offset (Uoff) may be provided by thecomparator 84; the voltage offset (Uoff) may be an input offset of thecomparator 84.

FIG. 8b show the voltages at points A and B and the Boolean values atoutputs H-J, of the MEMS micro-mirror device 80 shown in FIG. 8a when anshort circuit occurs. At time t_(o) the MEMS micro-mirror device 80undergoes normal operation. During normal operation one of voltages atpoints A and B will always be greater than the other; accordingly theoutputs H and I will always have opposite phase; specifically, when theoutput H is ‘1’ the output I will be ‘0’ during normal operation.Accordingly the output J of the XNOR gate will always be ‘0’ duringnormal operation of the MEMS micro-mirror device 80.

At time t₁ a short circuit occurs in the conduction coil 28 of the MEMSmicro-mirror 22 which causes the MEMS micro-mirror 22 to stoposcillating. The occurrence of the short circuit will cause both thevoltage at point A and the voltage at point B to both go to V_(DD), asis illustrated in FIG. 8b . The voltages at points A and B will thusbecome equal, and the outputs H,I of both first and second comparators83,84 will both go to ‘1’; accordingly the output J of the XNOR gate 85will change from ‘0’ to ‘1’. The detection of an short circuit in theMEMS micro-mirror device 80 can be thus be achieved by monitoring theoutput J of the XNOR gate 85 for when its Boolean value changes from ‘0’to ‘1’.

The MEMS micro-mirror device 80 may further comprise a control unitwhich is operatively connected to the output J of the XNOR gate 85, andwhich is configured to automatically shut-down the laser light source,and/or the laser driver and/or the MEMS micro-mirror device 80 when theoutput J of the XNOR gate 85 changes from ‘0’ to ‘1’.

FIG. 9a shows a MEMS mirror device 90 according to a further embodimentof the present invention in which a short circuit can be detected. TheMEMS micro-mirror device 90 comprises a circuit 91 which has all of thefeatures of the circuit 20 shown in FIG. 2a and like features areawarded the same reference numbers.

The MEMS micro-mirror device 90 further comprises a means for comparing92 which is configured to compare the voltages at first and second sides58 a, 58 b of the MEMS micro-mirror 22 to each other, so that an opencircuit in the MEMS mirror device 50 can be detected. Specifically, themeans for comparing 92, compares the voltage at point A to the voltage aB, so that an open circuit in the MEMS mirror device 90 can be detected.

The means for comparing 92 comprises a single XNOR gate 95 which has anoutput (J). The XNOR gate 95 comprises a ground pin 96, which iselectrically connected to point C so that the ground pin 96 of the XNORgate 95 is at kept at the same potential as the voltage across thecurrent source 23. Optionally a buffer may be electrically connectedbetween the point C and the ground pin 96. The XNOR gate 95 comprises adrive pin 97 which is connected to the power source (V_(DD)).

If the voltage UM is greater than the transition of an NMOS transistor,the detection circuit can be a XNOR gate, with its ground connected tothe point C.

A first input 98 a of the XNOR gate 95 is electrically connected to thefirst side 58 a of the MEMS micro-mirror 22, a second input 98 b of theXNOR gate 95 is electrically connected to the second side 58 a of theMEMS micro-mirror 22. So the voltage at the first side 58 a of the MEMSmicro-mirror 22 (i.e. the voltage at point A) and the voltage at thesecond side 58 b of the MEMS micro-mirror 22 (i.e. the voltage at pointB) both form the inputs to the XNOR gate 95.

FIG. 9b illustrates the voltages at points A and B and the Booleanvalues at output J of the XNOR gate 95, when a short circuit occurs. Attime t_(o) the MEMS micro-mirror device 90 undergoes normal operation.During normal operation the voltage at A and B will always be oppositein phase (i.e. A and B can have only two values: Vdd or Vdd−Um; so if Ais at Vdd, B will be at Vdd−Um, and if A is at Vdd−Um, B will be atVdd); when current follows along the first route through the H-bridgecircuit 91 then the voltage at A is equal to V_(DD) and the voltage at Bis equal to V_(DD)−Um; when current follows along the second routethrough the H-bridge circuit 91 then the voltage at A is equal toV_(DD)−Um the voltage at B is equal to V_(DD) (wherein Um is the voltagedrop across the MEMS micro-mirror 22) as is illustrated in the graphsFIG. 9b . Accordingly the output J of the XNOR gate 95 will always be‘O’ during normal operation of the MEMS micro-mirror device 80.

At time t₁ a short circuit occurs in the conduction coil 28 of the MEMSmicro-mirror 22 which causes the MEMS micro-mirror 22 to stoposcillating. The occurrence of the short circuit will cause both thevoltage at point A and the voltage at point B to both go to V_(DD), asis illustrated in FIG. 9b . The voltages at points A and B will thusbecome equal, and the output J of the XNOR gate 95 will thus change from‘0’ to ‘1’. The detection of a short circuit in the MEMS micro-mirrordevice 80 can be thus be achieved by monitoring the output J of the XNORgate 95 for when its Boolean value changes from ‘0’ to ‘1’.

The MEMS micro-mirror device 90 may further comprise a control unitwhich is operatively connected to the output J of the XNOR gate 95, andwhich is configured to automatically shut-down the laser light source,and/or the laser driver and/or the MEMS micro-mirror device 90 when theoutput J of the XNOR gate 95 changes from ‘0’ to ‘1’.

FIG. 10a shows a MEMS mirror device 100 according to a furtherembodiment of the present invention in which a short circuit, as well asa partial short circuit, can be detected.

A Short circuit in the conduction coil 23 occurs when the resistance ofthe conduction coil 23 is close to, or equal to, 0Ω (Ohm); this canoccur if conducting material with very low resistance electricallyconnects points A and B, thus shorting out the conduction coil 23. Apartial short circuit is when the resistance of the conduction coil 23is decreasing because parts of the conduction coil 23 are beingelectrically connected to each other. The problem with a partial shortcircuit is that for the same current conducted in the conduction coil23, the Lorentz force acting on the MEMS micro-mirror 22 will be loweredand thus the amplitude of oscillation of the MEMS micro-mirror 22 willdecrease and the angle over which the light beams are scanned willdecrease. A short circuit or a partial short circuit can happen forexample due to electro-migration in the conduction coil 23, ormechanical deformation of the conduction coil 23, leading to physicalcontact between different parts of the conduction coil 23.

The MEMS micro-mirror device 100 comprises a circuit 101 which has allof the features of the circuit 20 shown in FIG. 2a and like features areawarded the same reference numbers. The MEMS micro-mirror device 90further comprises a means for comparing 102.

The means for comparing 102 is configured to compare voltage across thecurrent source 23 (i.e. the voltage at point C) to a reference voltageto detect a short circuit in the MEMS mirror device. In this particularexample the reference voltage is the average value of the voltage acrossa current source 23 over a predefined period of time (i.e. the averageof the voltage at point C over a predefined period of time).

Specifically, the means for comparing 102 comprises, a comparator 103which has a first and second input 106,107 and an output F; and a lowpass filter 104 which is electrically connected to a junction (i.e.point C) which is between the H bridge circuit 101 and current source 23(i.e. the low pass filter 104 is electrically connected to point C). Thelow pass filter 104 is grounded (i.e. connected to gnd) or is connectedto Vdd. A buffer 109 is further electrically connected between saidjunction (i.e. point C) and capacitor 104, as is illustrated in FIG. 10a. The buffer 109 will ensure that parasitic voltage from the low passfilter 104 will not affect the voltage across the current source 23(i.e. the voltage at point C). In the present invention the low passfilter 104 comprises a capacitor, but it will be understood that the lowpass filter 104 may comprise any other suitable configuration orcombination of components e.g. a combination of capacitors andresistors. The average of the voltage at point C over a predefinedperiod of time is achieved by the low pass filter 104; the period oftime depends on the time constant of the low pass filter 104. The lowpass filter 104 is preferably designed in order to attenuate thevariations due to the induced voltage and therefore provide a singleconstant signal with a constant value, instead of a sinus shape signal(which is the case of the induced voltage signal at point C). Forexample, to have 20 dB attenuation, a first order low pass filter needsa resistor (which has an electrical resistance R) and a capacitance(which has an electrical capacitance C) where the average period is tentimes the MEMs mirror oscillation period [i.e. 10*MEMS mirroroscillation period=2*pi*R*C]. All the MEMS mirror oscillations whichhave a higher frequency than ½″*pi*R*C are attenuated therefore only theaverage value remains after the filter. A higher order filter can beused, with the benefit of reducing the ripple on the average valuesignal.

The first input 106 of the comparator 103 is electrically connected tothe junction (i.e. point C) between the H bridge 101 and current source23 so that the first input 106 to the comparator 103 is at a voltagewhich is equal to the voltage across the current source 23 (i.e. thevoltage at point C is applied to the first input 106 of the comparator103). The second input 107 to the comparator 103 is electricallyconnected to the low pass filter 104 so that the second input 107 to thecomparator 103 is at a voltage which is equal to the voltage across thelow pass filter 104 (i.e. the voltage at point E). The low pass filter104 can store previous voltages that were across the current source 23(i.e. can store the average voltage value of the point C, witheventually a residual oscillation depending on the time constant of thelow pass filter 104). The voltage across the low pass filter 104 (i.e.the voltage at point E) is therefore equal to the average value for thevoltage across a current source 23 over a predefined period of time. TheMEMS mirror device 100 further comprises a voltage offsetting means 111for providing a voltage offset (Uoff) to the voltage at point E in thecomparator 103. Alternatively the voltage offset (Uoff) may be providedby the input to the comparator 103 i.e. as an input offset of thecomparator 103. The voltage offset (Uoff) ensures that factors such astemperature change in of the MEMS mirror device 100 will not cause thevoltage across the current source 23 (i.e. the voltage at point C) toexceed the voltage at E plus the voltage offset; in other words theoffset (Uoff) will ensure that the voltage at point C will be largerthan ‘Voltage at E’+‘Uoff’ during normal operation when there is noshort circuit or partial short circuit in the MEMS mirror device 100).

FIG. 10b illustrates the voltages at points C and E; and alsoillustrates the voltage at E plus offset (Uoff). At time t_(o) the MEMSmicro-mirror device 100 undergoes normal operation. During normaloperation the voltage the value ‘Voltage at E’+‘Uoff’ at the secondinput 107 of the comparator 103 will always be larger than the voltageat point C at the first input 106 to the comparator 103. Therefore theoutput F of the comparator will be ‘0’.

At time t₁ a partial short circuit occurs in the conduction coil 28 ofthe MEMS micro-mirror 22 which causes resistance of the conduction coil28 to decrease and thus the voltage across the MEMS micro-mirror 22 todecrease. As the voltage across the MEMS micro-mirror 22 to decreasesthe voltage across the current source 23 (i.e. the voltage at point C)will correspondingly increase. At time t₂ the voltage across the currentsource 23 (i.e. the voltage at point C) has increased to the point whereit exceeds the value ‘Voltage at E’+‘Uoff’; thus the voltage at point Cat the first input 106 to the comparator 103 will now exceed the value‘Voltage at E’+‘Uoff’ at the second input 107 of the comparator 103.Accordingly the output F of the comparator 103 will change from ‘0’ to‘1’.

The MEMS micro-mirror device 100 may further comprise a control unitwhich is operatively connected to the output F of comparator 103, andwhich is configured to automatically shut-down the laser light source,and/or the laser driver and/or the MEMS micro-mirror device 100 when theoutput F of comparator 103 changes from ‘0’ to ‘1’.

By crossing the inputs of the comparator, the circuit shown on FIG. 10may detect if the MEMs micro-mirror 22 becomes blocked so that it nolonger oscillates about its oscillation axis. This is because once theMEMS micro-mirror 22 becomes blocked, the temperature of the MEMSmicro-mirror 22 will increase dramatically since there is no air coolingeffect due to oscillation. The increase in temperature will cause adecrease in the voltage across the current source 23 (i.e. causes anincrease in the voltage Um, so a decrease in the voltage at point C) tothe point where the value ‘Voltage at C’+‘Uoff’ falls below the voltageat point E; thus the voltage at point C+Uoff at the input 107(understood that the offset ‘Uoff’ is built into the comparator) of thecomparator 103 will now fall below the voltage at point Eat the input106 of the comparator 103. Accordingly the output F of the comparator103 will change from ‘0’ to ‘1’.

FIG. 11 shows a MEMS mirror device 120 according to a further embodimentof the present invention in which a short circuit, as well as a partialshort circuit, can be detected. The MEMS micro-mirror device 120comprises a circuit 121 which has all of the features of the circuit 20shown in FIG. 2a and like features are awarded the same referencenumbers.

The MEMS micro-mirror device 120 further comprises a means for comparing122. The means for comparing 122 is configured to compare voltage acrossthe current source 23 (i.e. the voltage at point C) to a referencedigital value representative of a reference voltage, to detect a shortcircuit, partial short circuit or a blockage in the MEMS mirror device.Specifically, the means for comparing 102 comprises, ananalogue-to-digital converter 125 which is electrically connected to thejunction between the H bridge circuit 121 and current source 23 (i.e.electrically connected to point C) so that the analogue-to-digitalconverter 125 can convert the voltage across the current source 23 (i.e.the voltage at point C) to a digital value. The analogue-to-digitalconverter 125 will therefore a digital value for the voltage at point Cat its output 128. The means for comparing 102 further comprises acomparator 127 which has an output 129. The comparator 127 is arrangedto compare the digital value for the voltage at point C, to a referencedigital value representative of a reference voltage. The referencedigital value is predefined and stored in a memory 124 which thecomparator 127 can access; the reference digital value may be stored inthe memory 124 at the manufacturing stage. If the digital value for thevoltage at point C is less than the reference digital valuerepresentative of a reference voltage, then the output 129 of thecomparator 127 will be ‘0’; and if the digital value for the voltage atpoint C is greater than the reference digital value representative of areference voltage, then the output 129 of the comparator 127 will be ‘1’(or vice versa).

During normal operation of the MEMS micro-mirror device 120 the digitalvalue for voltage at point C will always be less than the referencedigital value representative of a reference voltage. Accordingly theoutput 129 of the comparator 127 will be a Boolean value ‘0’.

If a short or partial short circuit occurs in the conduction coil 28 ofthe MEMS micro-mirror 22 this will cause the resistance of theconduction coil 28 to decrease and thus cause the voltage across theMEMS micro-mirror 22 to decrease. As the voltage across the MEMSmicro-mirror 22 decreases the voltage across the current source 23 (i.e.the voltage at point C) will correspondingly increase. As the voltage atpoint C increases the digital value for the voltage at point C which isoutput from the analogue-to-digital converter 125 will increase so thatit exceeds the reference digital value representative of a referencevoltage. Accordingly the output 129 of the comparator 127 will changefrom a Boolean value ‘0’ to a Boolean value ‘1’. The occurrence of ashort or partial short can thus be detected by monitoring for a changein the output 129 of the comparator 127 e.g. from a Boolean value ‘0’ toa Boolean value ‘1’ (or from a Boolean value ‘1’ to a Boolean value‘0’).

The MEMS micro-mirror device 120 may further comprise a control unitwhich is operatively connected to the output 129 of the comparator 127,and which is configured to automatically shut-down the MEMS micro-mirrordevice 120 when the output 129 of comparator 127 changes e.g. from aBoolean value ‘0’ to a Boolean value ‘1’ (or from a Boolean value ‘1’ toa Boolean value ‘0’).

It will be understood that the embodiments shown in FIG. 11 may also beused to detect if the MEMS micro-mirror 22 becomes blocked so that it nolonger oscillates about its oscillation axis. This is because once theMEMS micro-mirror 22 becomes blocked, the temperature of the MEMSmicro-mirror 22 will increase dramatically since there is no air coolingeffect due to oscillation. The increase in temperature will cause anincrease in the voltage across the current source 23 (i.e. causes anincrease in the voltage at point C). As the voltage at point C increasesthe digital value for the voltage at point C which is output from theanalogue-to-digital converter 125 will increase so that it exceeds thereference digital value representative of a reference voltage.Accordingly the output 129 of the comparator 127 will change from aBoolean value ‘0’ to a Boolean value ‘1’. In this case it preferable toknow in advance the value for the resistance of the conduction coil 23for various different temperatures so that account can be taken for thechanges in the resistance of the conduction coil 23 due to temperaturechanges in the MEMS micro-mirror device 120; a method of the presentinvention may therefore include the step of measuring the the resistanceof the conduction coil 23 at a plurality of different temperatures.

In this embodiment the digital value for the voltage at point C iscompared to the reference digital value representative of a referencevoltage which is stored in a memory. The electrical resistance of theconduction coil 23 changes due to the temperature, a new referencedigital value representative of a new reference voltage shouldpreferably be used for the comparison. This new reference voltage valuecan be determined by knowing the relation between the temperature andthe coil resistance (then using the Ohm's law U=RI relationship tocalculate U as function of the coil resistance (R) value, the currentpassing through the coil (I) is set to remain constant whatever thetemperature, and the voltage (U) at the mirror ends. A table listing theresistance of the conduction coil 23 at different temperatures can begenerated in a calibration step in which the resistance of theconduction coil 23 is measured at different temperatures.

FIG. 12 shows a MEMS mirror device 130 according to a further embodimentof the present invention in which a short and open circuit can bedetected.

The MEMS micro-mirror device 130 comprises a circuit 131 which has allof the features of the circuit 20 shown in FIG. 2a and like features areawarded the same reference numbers. The MEMS micro-mirror device 130further comprises a means for comparing 132.

The means for comparing 132 is configured to compare voltage across thecurrent source 23 (i.e. the voltage at point C) to an reference voltageto detect a short circuit in the MEMS mirror device. Specifically, themeans for comparing 132 comprises a first and second comparators 135 a,bwhich comprise outputs D and E respectively. A second input 136 b of thefirst comparator 135 a is electrically connected to a junction betweenthe H bridge 131 and current source 23 so that the second input 136 bhas a voltage equal to the voltage across the current source 23 (i.e.the second input 136 b has a voltage equal the voltage at point C), anda first input 136 a of the first comparator 135 a is provided with afirst reference voltage (Vrefp). A first input 137 a of the secondcomparator 135 b is electrically connected to a junction between the Hbridge 131 and current source 23 so that the first input 137 a has avoltage equal to the voltage across the current source 23 (i.e. thefirst input 137 a has a voltage equal to the voltage at point C), and asecond input 137 b of the second comparator 135 b is provided with asecond reference voltage (Vrefn).

In order to provide the first (Vrefp) and second (Vrefn) referencevoltages the means for comparing 132 comprises a first, second and thirdresistor 139 a-c, which are connected in series, and voltage (V_(DD)) isapplied across the first, second and third resistors 139 a-c such thatthe voltage at a junction 140 between the first and second resistors139,b define the first reference voltage (Vrefp) and the voltage at ajunction 142 between the second and third resistors 139 b,c define thesecond reference voltage (Vrefn). Alternatively the first (Vrefp) andsecond (Vrefn) reference voltages can each be provided by outputs of afirst and second digital convertor respectively; the first (Vrefp) andsecond (Vrefn) reference voltages can be changed more easily beadjusting the digital inputs to the first and second digital convertors.During normal operation of the MEMS micro-mirror device 130 the voltageat point C is always larger than the first and second referencevoltages, therefore the outputs D,E of the first and second comparators135 a,b will be Boolean value ‘1’. In the case an short circuit in theMEMS micro-mirror device 130 occurs the voltage at point C will exceedthe first reference voltage (Vrefp) and the output D of the firstcomparator 135 a changes from Boolean value ‘1’ to Boolean value ‘0’.When the short circuit occurs the resistance of the MEMS mirror 22decreases, the voltage value across the MEMS micro mirror 22 (Um)decreases. So the voltage value at point C will increase, and exceed thevalue of the first reference voltage (Vrefp). The value of the firstreference voltage (Vrefp) is determined by the maximum voltage value inpoint C in normal operation. In the case an open circuit in the MEMSmicro-mirror device 130 the voltage at point C will drop to less thanthe second reference voltage (Vrefn) and the output E of the secondcomparator 135 b changes from Boolean value ‘1’ to Boolean value ‘0’. Incase of open circuit, no current can drive through the MEMS micro-mirror22. The voltage value of point C will drop to 0, and drop less than thereference voltage (Vrefp). Thus the occurrence of a short circuit in theMEMS micro-mirror device 130 can be detected by monitoring for a changein the output D of the first comparator 135 a changes from Boolean value‘1’ to Boolean value ‘0’, and the occurrence of an open circuit in theMEMS micro-mirror device 130 can be detected by monitoring for a changein the output E of the second comparator 135 b changes from Booleanvalue ‘1’ to Boolean value ‘0’.

The MEMS micro-mirror device 130 may further comprise a control unitwhich is operatively connected to the outputs D,E of the first andsecond comparators 135 a,b respectively. The control unit is preferablyconfigured to automatically shut-down the MEMS micro-mirror device 130when either of the outputs D,E of the first and second comparators 135a,b change e.g. from a Boolean value ‘0’ to a Boolean value ‘1’ or froma Boolean value ‘1’ to a Boolean value ‘0’.

Advantageously, in the present invention none of the variables used todetect an open or short circuit are unaffected by temperature, thusproviding for more reliable detection of an open or short circuit.Furthermore, the time required to detectan open or short circuit dependsonly on the gain of the comparator, so fast detection is achievable.

It will be understood that in the context of the present invention“normal operation” is when the MEMS micro-mirror device operates withoutany short or open circuits. It will also be understood that in the aboveexamples the detection of an open or short circuit was based on a changein an output of the device from a Boolean value ‘0’ to a Boolean value‘1’; it will be understood that the circuits of the above-mentioneddevice could alternatively be configured to have an output which changesfrom a Boolean value ‘1’ to a Boolean value ‘0’ when an open or shortcircuit occurs, for example by reversing the inputs to comparators i.e.by changing the input to the positive input of the comparator to be theinput to the negative input of the comparator and by changing the inputto the negative input of the comparator to be the input to the positiveinput of the comparator. What is important is a state change; any statechange is acceptable i.e. the comparator may be designed to either gofrom ‘0’ to ‘1’ or from ‘1’ to ‘0’ to indicate the presence of the openor short circuit.

In the above example the voltage offsets (Uoff′) are positive voltageoffsets. However it will be understood that the voltage offsets (Uoff)may alternatively be a negative voltage offset. In the case of anegative voltage offset, the inputs to comparators should be reversedand the negative voltage offset is provided to the input to the negativeinput of the comparator.

It will also be understood that the comparators may be alternatively beconfigured to compare currents instead of voltages.

As an alternative to the comparators used in the above-mentionedexamples an ADC may be used to perform the comparisons. In other wordsthe means for comparing in each device may comprise a analogue todigital converter.

1-15. (canceled)
 16. An apparatus comprising: a microelectromechanicalsystem (MEMS) mirror comprising: a mirror; and a conduction coil toconduct a current and apply a force to the mirror to oscillate themirror about at least one axis; a power supply circuit electricallycoupled to the MEMS mirror, the power supply circuit to selectivelyapply voltage to a first side of the conduction coil or a second side ofthe conduction coil; a current source electrically coupled to the powersupply circuit; and a comparator to compare a voltage at the first andsecond side of the conduction coil to a voltage across the currentsource to detect an open circuit in the MEMS mirror.
 17. The apparatusof claim 16, the power supply circuit comprising an H bridge circuit.18. The apparatus of claim 17, the comparator comprising: an OR gatehaving at least a first and a second input; a first comparator coupledto the first input of the OR gate; and a second comparator coupled tothe second input of the OR gate.
 19. The apparatus of claim 18, whereinthe voltage at the first side of the conduction coil and the voltageacross the current source are inputs to the first comparator and thevoltage at the second side of the conduction coil and the voltage acrossthe current source are inputs to the second comparator.
 20. Theapparatus of claim 18, the current source comprising a first and asecond transistor electrically connected in series, wherein the voltageat the first side of the conduction coil and the voltage at a junctionbetween the first and second transistors are inputs to the firstcomparator and the voltage at the second side of the conduction coil andthe voltage at the junction between the first and second transistors areinputs to the second comparator.
 21. The apparatus of claim 17, thecurrent source comprising a first and a second transistor electricallyconnected in series, the comparator to compare a voltage at the firstand second side of the conduction coil to a voltage at a junctionbetween the first and second transistors.
 22. The apparatus of claim 21,wherein the voltage across the first and second transistors and thevoltage at a junction between the first and second transistors areinputs to the comparator.
 23. The apparatus of claim 17, the comparatorcomprising: an XNOR gate having at least a first and a second input; afirst comparator coupled to the first input of the XNOR gate; and asecond comparator coupled to the second input of the XNOR gate, whereinthe voltage at the first side of the conduction coil and the voltage atthe second side of the conduction coil are inputs to the firstcomparator and the voltage at the first side of the conduction coil andthe voltage at the second side of the conduction are inputs to thesecond comparator.
 24. The apparatus of claim 17, the comparator tocompare the voltage across the current source to a reference voltage todetect an open circuit in the MEMS mirror.
 25. The apparatus of claim17, the comparator comprising: a buffer electrically connected to ajunction between the H bridge and the current source; and a low passfilter which is electrically connected to the junction between the Hbridge and current source, wherein the comparator to compare an outputof the buffer to an output of the low pass filter.
 26. A methodcomprising: determining a first voltage at a first side of a conductioncoil of a microelectromechanical system (MEMS) mirror, the conductioncoil to conduct a current and apply a force to a mirror element of theMEMS mirror to oscillate the mirror element about at least one axis;determining a second voltage at a second side of the conduction coil;comparing the first voltage and the second voltage; and detecting achange in the comparison to detect an open circuit in the MEMS mirror.27. The method of claim 26, the conduction coil to be driven by acurrent source comprising a first and a second transistor operablycoupled in series, the method comprising: comparing a voltage across thefirst and second transistors to a voltage at a junction between thefirst and second transistors; and detecting a change in comparison ofthe voltage across the first and second transistors to the voltage at ajunction between the first and second transistors to detect an opencircuit in the MEMS mirror.
 28. The method of claim 26, the conductioncoil to be driven by a current source, the method comprising: comparinga voltage across the current source to a reference voltage; anddetecting a change in comparison of the voltage across the currentsource to the reference voltage to detect an open circuit in the MEMSmirror.
 29. The method of claim 28, wherein the reference voltage is anaverage value of the voltage across the current source over a predefinedperiod of time.
 30. A system comprising: a light source to emit a lightbeam; a microelectromechanical system (MEMS) mirror, the MEMS mirrorcomprising: a mirror to receive the light beam and reflect the lightbeam; and a conduction coil to conduct a current and apply a force tothe mirror to oscillate the mirror about at least one axis; a powersupply circuit electrically coupled to the MEMS mirror, the power supplycircuit to selectively apply voltage to a first side of the conductioncoil or a second side of the conduction coil; a current sourceelectrically coupled to the power supply circuit; and a comparator tocompare a voltage at the first and second side of the conduction coil toa voltage across the current source to detect an open circuit in theMEMS mirror.
 31. The system of claim 30, comprising a controller to senda control signal to the light source to turn off the light based on adetection of an open circuit in the MEMS mirror.
 32. The system of claim30, the power supply circuit comprising an H bridge circuit.
 33. Thesystem of claim 32, the comparator comprising: an OR gate having atleast a first and a second input; a first comparator coupled to thefirst input of the OR gate; and a second comparator coupled to thesecond input of the OR gate.
 34. The system of claim 32, the comparatorcomprising: an XNOR gate having at least a first and a second input; afirst comparator coupled to the first input of the XNOR gate; and asecond comparator coupled to the second input of the XNOR gate, whereinthe voltage at the first side of the conduction coil and the voltage atthe second side of the conduction coil are inputs to the firstcomparator and the voltage at the first side of the conduction coil andthe voltage at the second side of the conduction are inputs to thesecond comparator.
 35. The system of claim 32, the comparatorcomprising: a buffer electrically connected to a junction between the Hbridge and the current source; and a low pass filter which iselectrically connected to the junction between the H bridge and currentsource, wherein the comparator to compare an output of the buffer to anoutput of the low pass filter.